The Hordes of the Underdark campaign is the official campaign that was shipped with the Hordes of the Underdark expansion pack. Through this campaign, the player takes on the character of a hero of Waterdeep, questing to save the city from an invasion by residents of the Underdark (who arrive in the city by way of Undermountain). This hero is assumed to be the same character who completed the Shadows of Undrentide campaign, although players are free to use different characters. This is an advanced campaign, so characters starting under level 12 are immediately promoted to level 15.
The campaign is designed as a single-player adventure. Support for multiplayer was dropped early in its development cycle, giving rise to several potential problems if played with multiple people. (The problems are not necessarily game-breaking, though, and players have developed workarounds for most of them.)